Gelatin Skin Problem

I make a gelatin dessert from Knox gelatin, cherry juice that I reconstitute from concentrate and banana or mango. I have also used lemonade as the liquid.

I use 2 envelopes of gelatin powder to 2 1/2 cups of juice which I sweeten with NOW liquid stevia extract. One batch is 2 portions.

The problem, such as it is, is that I get a rubbery skin on the bottom of the mold. My mold is a round tupperware which I cover and put in the fridge after the juice-gelatin solution has cooled to room temperature.

How can I not get that rubbery skin?

I am using the gelatin as a medicine to reduce muscle soreness from swimming. My wife likes the gelatin for its flavor and texture.